Using the TI PAL modulator...
About the time the TI came out TV sets sold in the UK began to work well with a either a 50Hz or a 60Hz signal, as the tv sets were able to lock onto the frequencies well. My present CRT tv set is perfectly happy with an input of 50 or 60Hz.
There was a long standing issue in the UK with television sets made for the UK market not working too well with the UK TI99/4a. You could tune the TI sound in perfectly OR you could tune the TV picture in well. To get something approaching good meant VERY fine tuning of the tv channel, and if you got the sound perfect with no hum, the picture was probably rolling or there was colour loss etc.
This was designed in by TI. The PAL modulators were made for European PAL. The BBC chose to have their own version of PAL for the UK standard and there was a wider audio bandwidth. Or to put it another way, on the TI modulators of the day the sound channel was too close to the picture channel to play nicely with UK tv sets....
The UK used PAL STANDARD I while Europe used PAL STANDARD B or G - B and G used different bandwidths but the distance between image and sound was the same. The BBC wanted better quality sound so the sound channel in standard I was 0.50 MHz adrift of Standards B and G.,
I suspect this may be your problem with the RF signal. There is no ready answer except to tweak the fine tuning on the tv set- most sets had a manual fine tune opton. And compromise. Most folks preferred a good picture.
There's no fine tuning on this TV... just a channel detector. I ran that, it found 33 and 34 for the TI, and a bunch of digital channels. (Vizio, if it helps.)
Also, I think the fact it's B/W with splotchy artifacts of color is indicative that the color bands aren't being handled properly. Per "system information" it considers the channel 34 on antenna to be NTSC. So it's clearly NOT detecting it's a PAL signal or compensating. And no, there's no manual option to change it.
I think my last test here will be to acquire a PAL TV if I can (I'll know by Monday if I can get one), and test on that. It will require me to buy a transformer/converter for the TV.